Used as a DC voltage power supply, a typical Lithium-Ion (Li-Ion) battery pack usually includes a group of battery cells connected in series.
Charging and discharging the battery pack through normal operation over time may result in cell-to-cell variations in cell voltages. When one or more cells in a series string charge faster or slower than the others, an unbalanced condition may occur.
FIG. 1 illustrates a conventional cell balancing circuit using a dedicated pin to control an external bypass path. The positive terminal (anode) of a cell 102 is coupled to a controller 110 at terminal BAT1 through a first resistor 108. The negative terminal (cathode) of the cell 102 is coupled to the controller 110 at terminal BAT0 through a second resistor 106. An external bypass path is parallel-connected with the cell 102. The bypass path can include a current limiting resistor 101 and a bleeding control switch 104 connected in series with the current limiting resistor 101. The switch 104 is controlled by a controller 110 via a dedicated pin CB.
When an unbalanced condition occurs, for example, a voltage of cell 102 is greater than that of any other cell (not shown in FIG. 1 for purposes of brevity and clarity) in the battery pack, the controller 110 may turn on the switch 104 to enable a bleeding current to flow through the external bypass path, thereby balancing cell voltages in the battery pack. One of the disadvantages of this method is that an extra pin CB is needed to control the bleeding control switch 104, which can increase the cost.
FIG. 2 shows another conventional cell balancing circuit using an internal switch to control a bleeding control switch. Elements labeled the same as in FIG. 1 have similar functions and will not be repetitively described herein for purposes of brevity and clarity. In the controller 210, an internal switch 212 is coupled between terminal BAT1 and terminal BAT0, and is under control of an internal switch control unit 214 which is also located in the controller 210.
In FIG. 2, a voltage drop on the resistor 106 determines a conductance status of the bleeding control switch 104. Furthermore, when internal switch 212 is turned on by a control signal from the internal switch control unit 214, the voltage drop on the resistor 106 is determined by a voltage divider including resistor 108 and resistor 106. As such, the voltage drop on the resistor 106 may be small (e.g., half of the cell voltage).
There are also some disadvantages of this method. Firstly, since the voltage drop on the resistor 106 may be small, the threshold voltage of the bleeding control switch 104 may have to be low enough (e.g., 1V) such that the bleeding control switch 104 is able to be turned on by the small voltage drop across the resistor 106. If the bleeding control switch 104 is a MOSFET, then it may need to be a MOSFET with a lower threshold voltage. Such MOSFET is generally expensive, which will increase the total cost of the circuit.
Secondly, considering a group of series-connected cells, bleeding control switches of neighboring cells can not be simultaneously enabled, which leads to limited practical usage of the balancing circuit for a battery pack having a group of cells. In FIG. 2, in order to conduct the bypass path, internal switch 212 is turned on and a current flows from terminal BAT0, through the resistor 106 to the negative terminal of the cell 102. If there is a second cell (not shown in FIG. 2 for purposes of brevity and clarity) connected in series with cell 102, the resistor 106 is coupled between the positive terminal of the second cell and the controller 210. In order to conduct the bypass path of the second cell, a bleeding current needs to flow from a positive terminal of the second cell, through the resistor 106 to terminal BAT0, which may result in a confliction of the current direction.
Thirdly, the cell voltage may need to be high enough to ensure that the bleeding control switch 104 can be operable. If the cell voltage is too low, the gate-to-source voltage Vgs of the bleeding control switch 104 (that is, the voltage drop on the resistor 106) may never be greater than the threshold voltage of the bleeding control switch 104. As such, the switch 104 may not be turned on even if internal switch 212 is turned on. Consequently, this method may not be applicable for low voltage cells, such as LiFePo4 cells.